When oysters reproduce and grow in nature, at every stage of their development they are susceptible to predators and disease. The mortality rate during the larval stage is estimated to exceed 99%.
Oysters have been grown and harvested for a long time. In some locations cleaned, empty, oyster shells are immersed in waters where oyster larvae are known to occur naturally. The larvae settle on the oyster shells which are in effect an artificial substrate; this is known as cultch. The juvenile oysters either attached to the cultch or removed from it are used as seed oysters for planting in oyster beds. Increasing demand for oysters has led to the development of processes for the more efficient culture of seed oysters suitable for planting in oyster beds to be grown to commercial maturity by any one of the various methods well known to those skilled in the art.
The artificial culture of seed oysters involves obtaining larvae from brood oysters, growing then in carefully controlled conditions and feeding them with appropriate algae. When they are at the eyed-larva stage they are induced to settle on an artifical cultch in a manner which is convenient to their subsequent culture, generally in beds in the sea or in an estuary.
Processes have been developed to produce cultch-free seed in the form of separate, individual animals unattached to any form of substrate. In such processes the eyed-larvae are induced to settle on a temporary cultch surface, such as a plastic or lead sheet as is disclosed is U.S. Pat. No. 3,526,209 or a mesh as disclosed in U.S. Pat. No. 3,495,573. After most of the larvae have set but before their shells become permanently attached to the cultch, they are washed or brushed from the temporary cultch assisted by a strong jet of water. This yields cultch-free spat which are then placed in spat trays where they are fed with appropriate algae. When the cultch-free spat have grown to such a size that they are capable of surviving in the open water situations they are used as seed for cultivation to maturity.
In British Pat. No. 1 366 394 a method of operating the spat trays involving bubble agitation of the culture medium is disclosed. This agitation causes the separate, cultch-free spat to be in a pseudo free swimming state even though the spat themselves are incapable of swimming under their own motive power. The potential advantages claimed for having the spat in this pseudo free-swimming state in the spat trays in contrast to the conventional stationary state on a suitable supporting tray were the rapid removal of potentially self-toxic metabolites the spats produce, the more efficient presentation of the food algae, and the inhibition of bacterial infection which can occur through the physical damage caused by the removal of the spat from the cultch.